The present invention relates to a magnetic recording medium and a manufacturing method therefor. More particularly, the invention relates to a doublesided, two layer vertical magnetization type recording medium and a manufacturing method therefor.
Recently, a vertical magnetization type recording system has been proposed in the art which uses a magnetic recording medium having an "easy" magnetizing axis perpendicular to its surface. In the vertical magnetization recording system, as the recording density increases, the diamagnetic field in the recording medium decreases, and an excellent reproduction output is provided. Thus, the system is fundamentally suitable for high-density recording operations.
In order to perform a magnetic recording operation according to the vertical magnetization type recording system, it is necessary to use a magnetic recording medium having an easy magnetizing axis perpendicular to its film surface. An example of such a vertical magnetization type recording medium can be provided by forming a Co-Cr film layer on a support of a macromolecular material or nonmagnetic material such as a nonmagnetic metal by sputtering or the like.
For the purpose of improving the recording and reproducing efficiency for vertical magnetization type recording and reproducing, a so-called two-film-layer vertical magnetization type recording medium has been proposed in which a high-magnetic-permeability layer of soft magnetic material such as a permalloy (Ni-Fe alloy) film layer is formed, as a base layer, under the vertical magnetization type recording layer such as the aforementioned Co-Cr alloy film layer.
For a flexible disk according to the aforementioned vertical magnetization type recording system, a so-called two-layer vertical magnetization type recording medium, on both sides of which the two-layer vertical magnetization type recording layer is formed, is suitable because it provides a large recording capacity and curling can be readily eliminated.
Heretofore, a sputtering apparatus as shown in FIG. 1 has been used to form the two-layer vertical magnetization type recording layer on the film-shaped support.
In this apparatus, a vacuum chamber 1 is evacuated through a discharge pipe 9. Argon gas is introduced into the vacuum chamber 1 thus evacuated in an amount so that the vacuum chamber 1 is maintained at 5.times.10.sup.-3 torr. In the vacuum chamber 1 thus treated, a film shaped support 5 is supplied from a support supply roll 3 and is subjected to sputtering while being wound on a cooling can 2.
That is, the film-shaped support 5, supplied from the support supply roll 3, is conveyed via the cooling can 2 to a support take-up roll 4 and is wound on the latter. In this operation, a permalloy target 6 and a Co-Cr target 7 arranged around the cooling can 2 form a double-film layer consisting of a permalloy film layer and a Co-Cr film layer on one side of the support 5.
Thereafter, the pressure in the vacuum chamber is restored to atmospheric pressure, and the film-shaped support is so set in the vacuum chamber that a similar double-film layer is formed on the other side thereof. After the vacuum chamber is evacuated again, a double-film layer is formed on the other side by sputtering in the same manner.
In another proposed method, a double-sided sputtering apparatus as shown in FIG. 2 is used to form the aforementioned double-film layer on each of the two sides of a film-like support in one conveyance. In accordance with this method, a permalloy target 31 and a Co-Cr target 32 form a double-film layer on one side of the film-shaped support, and a permalloy target 33 and a Co-Cr target 34 form a similar film layer on the other side.
More specifically, as shown in FIG. 2, first and second cooling cans 23 and 24 are provided in a vacuum chamber 21. After the vacuum chamber 21 is evacuated through a discharge pipe 28, argon gas is introduced into the vacuum chamber 21 in an amount so as to maintain a pressure of 5.times.10.sup.-3 torr. Under this condition, a film-shaped support 28 is supplied from a support supply roll 22, wound on the cooling cans 23 and 24, and wound on a take-up roll 25. At the cooling can 23, using the permalloy target 31 and the Co-Cr target 32 provided around it, a double-film layer consisting of a permalloy film layer and a Co-Cr film layer is formed on one side of the film-shaped support 28 by sputtering. In succession, at the cooling can 24, using the permalloy target 33 and the Co-Cr target 34 provided around it, another double film layer is formed on the other side of the support by sputtering.
In the case where a double-sided, two-layer vertical magnetization type recording medium is formed using a take-up type continuous sputtering apparatus as shown in FIG. 1 or 2, tension is, in general, applied in one direction of the film-shaped support. As a result, the static magnetic characteristic of the permalloy film layer becomes anisotropic; that is, the film layer surface becomes anisotropic in magnetic characteristic, and static magnetic characteristics such as magnetic permeability become different in different directions on the support. Therefore, when a recording or reproducing operation is carried out with a floppy disk formed by stamping (cutting) the recording medium, the reproduction output varies in the circumferential direction.
On the other hand, in the case where a doublesided, two-layer vertical magnetization type recording medium having Co-Cr film layers of high coercive force H.sub.c in the vertical direction is formed by a high-speed sputtering method such as a DC magnetron sputtering method, in general, it is preferable that the support be kept at a high temperature while forming the Co-Cr film layers by sputtering.
In a double-sided, two-layer vertical magnetization type recording medium formed using a film shaped support of macromolecular material as described above, when the film-shaped support is heated, it often occurs that the surface characteristic of the film-shaped support is deteriorated, or the two sides of the recording medium have different surface characteristics, vertical orientation, H.sub.c (vertical), etc. As a result, the recording and reproducing characteristics are deteriorated or made variable. This is a serious difficulty preventing the maintenance of desired characteristics of the recording medium during mass production thereof.
Accordingly, there has been a strong demand for provision of a double-sided, two-layer vertical magnetization type recording medium which has two sides of uniform characteristics and which is excellent as a vertical magnetization type recording medium, and also for a manufacturing method for such a recording medium.